1. Field of the Invention
This invention relates to the recovery of xylose and xylitol products from alkaline and acid treated extracts employed during the treatment of xylan-containing materials, especially hemi-celluloses of xylan-containing natural products. This invention is particularly concerned with maximizing the yield of xylose or xylitol by treatment of acid extract solutions and acid solutions obtained by the treatment of the material and aqueous wash solutions.
2. Discussion of the Prior Art
D-(+) xylose and its hydrogenation product xylitol have an appreciable technical significance. Xylose can be used, for example, in the foodstuffs industry for various purposes, while xylitol has proven to be a very good sweetening agent for diabetics. Almost exclusively leaf-bearing wood types, such as beech and chestnut, are used as starting material for the industrial production of xylose. The yields lie at 10-12% (see for example Ger. Pat. Sp. No. 912,440).
From Ger. Pat. Sp. No. 834,079 the obtaining of xylose from oat husks is known. In this process the oat husks are heated to boiling point with 0.08% ammonia or extracted with a benzene alcohol mixture. Then the used pressure hydrolysis takes place with 0.2 to 0.5% H.sub.2 SO.sub.4 at 125.degree. C. No further processing takes place.
In the preliminary tratment with NH.sub.3, 4 kg of NH.sub.3 as 0.08% solution are used on 1000 kg of oat husks. In order to split off the acetic acid, however 17 kg of NH.sub.3 would be necessary. Moreover under the conditions stated in the Ger. Pat. Sp. splitting off and thus removal of the acetic acid, which amounts to about 6% of the weight of the oat husks, could hardly take place.
On the basis of this process, in German patent applications Nos. 2,358,407 and 2,358,472, processes are proposed for the production of xylose solutions by decomposition of leaf-bearing wood or oat husks with a basic medium and treatment of the obtained solid residue with mineral acid. These processes, which are characterised in that alkali hydroxide is used as basic medium, firstly permit complete exploitation of the starting material and also produce a higher xylose yield.
In these processes the alkaline decomposition solution must be filtered off from the residue, whereupon the residue must be washed in order to remove the alkali as extensively as possible. An alkali content in the residue would be troublesome in the subsequent acid decomposition. The filtration and washing out of the residue occurring in the alkaline decomposition are carried out in large industrial operation generally with the aid of filter presses, which require relatively large quantities of washing water, so that waste water problems can arise. Moreover the recovery of the acetic acid present as alkali acetate causes difficulties in the case of dilute decomposition and washing solutions.
The residue occurrng in the second stage after the treatment with mineral acid is also ordinarily separated with the aid of filter presses from the acid, xylose-containing decomposition solution, after-washing likewise being necessary. Thus the xylose solutions occur in relatively dilute form, so that the energy consumption in concentration by evaporation becomes relatively great.
It is an object of the present invention to, avoid the disadvantages involved with filter presses, and to provide a process for the two-stage decomposition of hemi-celluloses in which the decomposition solutions are obtained in relatively concentrated form and the quantity of washing water at every stage can be kept to a minimum.